arXiv subjects

James F. E. Croft

Publications and source records attributed to James F. E. Croft.

At least 19 recordsLinked to original sources

arXiv2026

Converging on bound states in coupled-channel calculations

We develop a robust algorithm for locating bound states in coupled-channel calculations. Bound states exist at energies where an individual eigenvalue of a log-derivative or ratio matching matrix passes through zero. We describe an algorithm to identify the required eigenvalue of the matching matrix over the full range of energy where it exists. This allows much simpler programming than previous methods. We also consider the choice of the matching distance $R_\textrm{match}$, where the matching matrix is defined; coupled-channel methods are most efficient if $R_\textrm{match}$ is chosen to be in the classically allowed region for all channels that support bound states of interest, but not very close to a node in the wavefunction.

physics.chem-ph↗

arXiv2026

Long-range states in collisions of ultracold molecules

We use coupled-channel calculations to explore the nature of near-threshold bound states in a simplified model of Rb+KRb. This is a prototype for systems with very strong coupling at short range and chaotic behavior for the short-range states. We find that there are states with strong long-range character that exist close to threshold and probably persist to depths at least 100 GHz below each threshold. These states are only weakly coupled to the short-range states and do not form part of the chaotic manifold. Since they spend little time at short range, they are relatively insensitive to destruction by laser light. They can thus have long lifetimes that are unrelated to the density of states and can cause narrow Feshbach resonances when the states are shifted across thresholds by external fields.

physics.atom-ph↗

arXiv2023

Electro-association of ultracold dipolar molecules into tetramer field-linked states

The presence of electric or microwave fields can modify the long-range forces between ultracold dipolar molecules in such a way as to engineer weakly-bound states of molecule pairs. These so-called field-linked states [Avdeenkov et al., Phys. Rev. Lett. 90, 043006 (2003), Lassablière et al., Phys. Rev. Lett. 121, 163402 (2018)], in which the separation between the two bound molecules can be orders of magnitude larger than the molecules themselves, have been observed as resonances in scattering experiments [Chen et al., Nature 614, 59 (2023)]. Here, we propose to use them as tools for the assembly of weakly-bound tetramer molecules, by means of ramping an electric field, the electric-field analog of magneto-association in atoms. This ability would present new possibilities for constructing ultracold polyatomic molecules.

physics.atom-ph↗

arXiv2022

Cold collisions of rovibrationally excited D$_2$ molecules

The H$_2$+H$_2$ system has long been considered as a benchmark system for ro-vibrational energy transfer in bimolecular collisions. However, most studies thus far have focused on collisions involving H$_2$ molecules in the ground vibrational level or in the first excited vibrational state. While H$_2$+H$_2$/HD collisions have received wide attention due to the important role they play in astrophysics, D$_2$+D$_2$ collisions have received much less attention. Recently, Zhou et al. [Nat. Chem. 4 658 (2022)] examined stereodynamic aspects of rotational energy transfer in collisions of two aligned D$_2$ molecules prepared in the $v=2$ vibrational level and $j=2$ rotational level. Here, we report quantum calculations of rotational and vibrational energy transfer in collisions of two D$_2$ molecules prepared in vibrational levels up to $v=2$ and identify key resonance features that contribute to the angular distribution in the experimental results of Zhou et al. The quantum scattering calculations were performed in full dimensionality and using the rigid-rotor approximation using a recently-developed highly-accurate six-dimensional potential energy surface for the H$_4$ system that allows descriptions of collisions involving highly vibrationally excited H$_2$ and its isotopologues.

physics.chem-ph↗

arXiv2022

Microscopy of an ultranarrow Feshbach resonance using a laser-based atom collider: A quantum defect theory analysis

We employ a quantum defect theory framework to provide a detailed analysis of the interplay between a magnetic Feshbach resonance and a shape resonance in cold collisions of ultracold $\rm ^{87}Rb$ atoms as captured in recent experiments using a laser-based collider [Phys. Rev. Research 3, 033209 (2021)]. By exerting control over a parameter space spanned by both collision energy and magnetic field, the width of a Feshbach resonance can be tuned over several orders of magnitude. We apply a quantum defect theory specialized for ultracold atomic collisions to fully describe of the experimental observations. While the width of a Feshbach resonance generally increases with collision energy, its coincidence with a shape resonance leads to a significant additional boost. By conducting experiments at a collision energy matching the shape resonance and using the shape resonance as a magnifying lens we demonstrate a feature broadening to a magnetic width of 8 G compared to a predicted Feshbach resonance width $\ll 0.1$~mG.

cond-mat.quant-gas↗

arXiv2022

Stereodynamical control of cold collisions between two aligned D2 molecules

Resonant scattering of optically state-prepared and aligned molecules in the cold regime allows the most detailed interrogation and control of bimolecular collisions. This technique has recently been applied to collisions of two aligned ortho-D2 molecules prepared in the j=2 rotational level of the v=2 vibrational manifold using the Stark-induced adiabatic Raman passage technique. Here, we develop the theoretical formalism for collisions of two aligned molecules and apply our approach to state-prepared D2(v=2,j=2) + D2(v=2,j=2) --> D2(v=2,j=2) + D2(v=2,j=0) collisions. Quantum scattering calculations were performed in full-dimensionality on an accurate H$_2$-H$_2$ interaction potential. Key features of the experimental angular distributions are reproduced and attributed primarily to a partial wave resonance with orbital angular momentum L=4

physics.atom-ph↗

arXiv2022

The role of low energy resonances in the stereodynamics of cold He+D2 collisions

In recent experiments using the Stark-induced Adiabatic Raman Passage (SARP) technique, Zhou et al. measured the product's angular distribution for the collisions between He and aligned D2 molecules at cold collision energies. The signatures of the angular distributions were attributed to a l=2 resonance that governs scattering at low energies. A first principles quantum mechanical treatment of this problem is presented here using a highly accurate interaction potential for the He-H2 system. Instead, our results predict a very intense l=1 resonance at low energies, leading to angular distributions that differ from those measured in the experiment. A good agreement with the experiment is achieved only when the l=1 resonance is artificially removed, for example, by excluding the lowest energies present in the experimental velocity distribution. Our analysis revealed that neither the position nor the intensity of the l=1 resonance significantly changes when the interaction potential is modified within its predicted uncertainties. Energy-resolved measurements may help to resolve the discrepancy.

physics.chem-ph↗

arXiv2021

Anomalous Lifetimes of Ultracold Complexes Decaying into a Single Channel: What's Taking So Long in There?

We investigate the lifetimes of complexes formed in ultracold molecule collisions. Employing both transition-state-theory and an optical model approach we examine processes that can extend the lifetime of complexes beyond that predicted by Rice-Ramsperger-Kassel-Marcus theory. We focus on complexes that possess only one open channel, and find that the extreme distribution of widths for this case favors low decay rates. Thus decay from a complex into a single energetically available channel can be anomalously slow, and moreover nonexponential in time. We apply the theory to two systems of current experimental interest, RbCs and NaRb, finding qualitatively that the empirical time scales can be accounted for in the theory.

cond-mat.quant-gas↗

arXiv2021

Electric field dependence of complex-dominated ultracold molecular collisions

Recent experiments on ultracold non-reactive dipolar molecules have observed high two-body losses, even though these molecules can undergo neither inelastic, nor reactive (as they are in their absolute ground state), nor light-assisted collisions (if they are measured in the dark). In the presence of an electric field these losses seem to be near universal (the probability of loss at short-range is near unity) while in the absence of it the losses seem non-universal. To explain these observations we propose a simple model based on the mixing effect of an electric field on the states of the two diatomic molecules at long-range and on the density-of-states of the tetramer complex formed at short-range, believed to be responsible for the losses. We apply our model to collisions of ground-state molecules of endothermic systems, of current experimental interest.

physics.atom-ph↗

arXiv2021

Stereodynamic control of cold rotationally inelastic CO + HD collisions

Quantum control of molecular collision dynamics is an exciting emerging area of cold collisions. Co-expansion of collision partners in a supersonic molecular beam combined with precise control of their quantum states and alignment/orientation using Stark-induced Adiabatic Raman Passage allows exquisite stereodynamic control of the collision outcome. This approach has recently been demonstrated for rotational quenching of HD in collisions with H2, D2, and He and D2 by He. Here we illustrate this approach for HD(v=0,j=2)+CO(v=0,j=0) -> HD(v'=0,j')+CO(v'=0,j') collisions through full-dimensional quantum scattering calculations at collision energies near 1 K. It is shown that the collision dynamics at energies between 0.01--1K are controlled by an interplay of L=1 and L=2 partial wave resonances depending on the final rotational levels of the two molecules. Polarized cross-sections resolved into magnetic sub-levels of the initial and final rotational quantum numbers of the two molecules also reveal a significant stereodynamic effect in the cold energy regime. Overall, the stereodynamic effect is controlled by both geometric and dynamical factors, with parity conservation playing an important role in modulating these contributions depending on the particular final state.

physics.chem-ph↗

arXiv2020

Unified model of ultracold molecular collisions

A scattering model is developed for ultracold molecular collisions, which allows inelastic processes, chemical reactions, and complex formation to be treated in a unified way. All these scattering processes and various combinations of them are possible in ultracold molecular gases, and as such this model will allow the rigorous parametrization of experimental results. In addition we show how, once extracted, these parameters can be related to the physical properties of the system, shedding light on fundamental aspects of molecular collision dynamics.

physics.atom-ph↗

arXiv2020

Non-adiabatic quantum interference effects and chaoticity in the ultracold Li + LiNa $\to$ Li$_2$ + Na reaction

Electronically non-adiabatic effects play an important role in many chemical reactions. How these effects manifest in cold and ultracold chemistry remain largely unexplored. Here, through first principles non-adiabatic quantum dynamics calculations of the Li + LiNa $\to$ Li$_2$ + Na chemical reaction, it is shown that non-adiabatic dynamics induces quantum interference effects that dramatically alter the ultracold rotationally resolved reaction rate coefficients. The interference effect arises from a conical intersection between the ground and an excited electronic state that is energetically accessible even for ultracold collisions. These unique interference effects might be exploited for quantum control applications as a quantum molecular switch. A statistical analysis of rotational populations of the Li$_2$ product reveals a Poisson distribution implying an underlying classically chaotic dynamics. The Poisson distribution is robust and amenable to experimental verification and appears to be a universal property of ultracold reactions involving alkali metal dimers.

physics.chem-ph↗

arXiv2019

Stereodynamical control of a quantum scattering resonance in cold molecular collisions

Cold collisions of light molecules are often dominated by a single partial wave resonance. For the rotational quenching of HD(v=1,j=2) by collisions with ground state para-H2, the process is dominated by a single L=2 partial wave resonance centered around 0.1 K. Here, we show that this resonance can be switched on or off simply by appropriate alignment of the HD rotational angular momentum relative to the initial velocity vector, thereby enabling complete control of the collision outcome.

quant-ph↗

arXiv2018

Unravelling the stereodynamics of cold HD-H2 collisions

Measuring inelastic rates with partial wave resolution requires temperatures close to a Kelvin or below, even for the lightest molecule. In a recent experiment Perreault et al. [1] studied collisional relaxation of excited HD molecules in the v = 1, j = 2 state by para- and ortho-H2 at a temperature of about 1 K, extracting the angular distribution of scattered HD in the v = 1,j = 0 state. By state-preparation of the HD molecules, control of the angular distribution of scattered HD was demonstrated. Here, we report a first-principles simulation of that experiment which enables us to attribute the main features of the observed angular distribution to a single L = 2 partial-wave shape resonance. Our results demonstrate important stereodynamical insights that can be gained when numerically-exact quantum scattering calculations are combined with experimental results in the few-partial-wave regime.

physics.chem-ph↗

arXiv2014

Non-Sticking of Helium Buffer Gas to Hydrocarbons

Lifetimes of complexes formed during helium-hydrocarbon collisions at low temperature are estimated for symmetric top hydrocarbons. The lifetimes are obtained using a density-of-states approach. In general the lifetimes are less than 10-100 ns, and are found to decrease with increasing hydrocarbon size. This suggests that clustering will not limit precision spectroscopy in helium buffer gas experiments. Lifetimes are computed for noble-gas benzene collisions and are found to be in reasonable agreement with lifetimes obtained from classical trajectories as reported by Cui {\it et al}.

physics.atom-ph↗

arXiv2014

Long-Lived Complexes, Ergodicity and Chaos in Ultracold Molecular Collisions

Estimates for the lifetime of collision complexes formed during ultracold molecular collisions based on density-of-states arguments are shown to be consistent with similar estimate based on classical trajectory calculations. In the classical version, these collisions are shown to exhibit chaos, and their fractal dimensions are calculated versus collision energy. From these results, a picture emerges that ultracold collisions are classically ergodic, justifying the density-of-states estimates for lifetimes. These results point the way toward using the techniques of classical and quantum chaos to interpret molecular collisions in the ultracold regime.

physics.chem-ph↗

arXiv2012

Multichannel Quantum Defect Theory for cold molecular collisions with a strongly anisotropic potential energy surface

We show that multichannel quantum defect theory (MQDT) can be applied successfully as an efficient computational method for cold molecular collisions in Li+NH, which has a deep and strongly anisotropic interaction potential. In this strongly coupled system, closed-channel poles restrict the range over which the MQDT Y matrix can be interpolated. We present an improved procedure to transform the MQDT reference functions so that the poles are removed from the energy range of interest. Effects due to very long-range spin-dipolar couplings are outside the scope of MQDT, but can be added perturbatively. The new procedure makes it possible to calculate the elastic and inelastic cross sections needed to evaluate the feasibility of sympathetic cooling of NH by Li using coupled-channel calculations at only 5 combinations of energy and field.

physics.chem-ph↗

arXiv2012

Optimized Multichannel Quantum Defect Theory for cold molecular collisions

Multichannel quantum defect theory (MQDT) can provide an efficient alternative to full coupled-channel calculations for low-energy molecular collisions. However, the efficiency relies on interpolation of the Y matrix that encapsulates the short-range dynamics, and there are poles in Y that may prevent interpolation over the range of energies of interest for cold molecular collisions. We show how the phases of the MQDT reference functions may be chosen so as to remove such poles from the vicinity of a reference energy and dramatically increase the range of interpolation. For the test case of Mg+NH, the resulting optimized Y matrix may be interpolated smoothly over an energy range of several Kelvin and a magnetic field range of over 1000 G. Calculations at additional energies and fields can then be performed at a computational cost that is proportional to the number of channels N and not to N^3.

physics.chem-ph↗